This invention relates to a multilayer fired ceramic circuit board and a process for producing the same.
As multilayer circuit boards, there have been known those using glass fiber reinforced epoxy resin as substrates and those using ceramics such as alumina ceramics as substrates.
The former multilayer circuit boards are generally produced by laminating a plurality of alternate layers of glass-epoxy substrates and prepregs, said glass-epoxy substrates having conductive circuit patterns on the surfaces thereof and said prepregs made from a fibrous material impregnated with a thermosetting resin, bonding each other under pressure with heating, drilling holes, and conducting through hole plating. In such multilayer circuit boards, since thermal conductivity of the glass-epoxy substrate is small (about 7.times.10.sup.-4 cal/cm.sec..degree.C.), heat resistance of it is large and thus it has poor heat dissipation. Therefore, there is a disadvantage in that when integrated circuits, large scale integrated circuits, etc., having a large heat release value, are directly mounted on such a glass-epoxy substrate in a high density, there takes place an excessive temperature rise, which results in making driving impossible.
On the other hand, in the case of the ceramic substrates, since alumina having thermal conductivity of (5-8).times.10.sup.-2 cal/cm.sec..degree.C. is used as a substrate, the substrate has good heat dissipation and can mount large scale integrated circuits, etc., in a high density. Such multilayer circuit boards having ceramic substrates as a main constituent can be produced by the following ways:
(1) A process for producing a multilayer ceramic circuit board comprising laminating a plurality of green ceramic sheets (unfired ceramic sheets) having printed conductive pastes thereon, making them into a laminated unit with a hot pressing operation, and sintering at a temperature of 1500.degree. C. or higher (British Pat. No. 1,538,818: IBM Technical Disclosure Bulletin Vol. 21, No. 5, 1860-1861 (1978): ibid. Vol. 21, No. 5, 1862-1863 (1978), etc.).
(2) A process for producing a multilayer ceramic circuit board comprising printing alternatively a plurality of alternate layers of conductive paste of heat resistant metal such as tungsten, molybdenum, or the like and insulating paste of alumina series material on a green sheet and sintering these alternate layers (Denshi Zairyo No. 7, p. 59-63 (1974); Semiconductor/IC Conference Proceeding p. 71-77 (1972)).
(3) A process for producing a multilayer ceramic circuit board comprising printing alternatively a plurality of conductive layers and glass paste for forming insulating layers on a ceramic substrate and firing these alternate layers (Solid State Technology, No. 9, p. 63-66 (1978)).
According to the processes (1) and (2) mentioned above, since the procedure of sintering is conducted at 1500.degree. C. or higher after the printing procedure, there takes place dimensional shrinkage of 15 to 20%, which results in forming warp of the substrates and producing deviations of dimensional accuracy and lowering the yield remarkably. According to the process (3) mentioned above, since the procedure of firing a conductive layer and the procedure of firing an insulating layer on the fired conductive layer and the procedure of firing a conductive layer on the fired insulating layer are repeated, there is a limitation in the number of layers to be laminated and about 4 layers can be laminated at most together with a disadvantage in workability.
In addition, Japanese patent appln. Kokoku (Post-Exam Publn) No. 5374/75 discloses a process for producing a multilayer ceramic plate comprising disposing a fired ceramic thin plate having a glass ceramic paste layer on the underside thereof on a fired ceramic thin plate having wiring patterns and welding them, filling holes formed on the disposed fired ceramic thin layer with a conductive paste and at the same time forming wiring patterns on the surface thereof, and repeating the procedure of lamination mentioned above. According to this process, since the lamination of one ceramic plate and filling of holes with a conductive paste and forming wiring patterns should be conducted one after another, this process is very disadvantageous from the viewpoint of productivity, particularly in the case of producing a multilayer ceramic plate having 10 to 20 or more layers of substrates.
Still further, Japanese patent appln. Kokai (Laid-Open) No. 79268/77 discloses a process for producing a multilayer printed wiring board using inorganic insulating plates comprising sandwitching an intermediate layer plate of an inorganic insulating plate having adhesive layers on the both sides thereof and having through conductors made of a low-melting metal between two printed wiring boards made of inorganic insulating plates having wiring patterns thereon, and forming a laminated unit from said two printed wiring boards sandwitching the intermediate layer plate by applying heat and pressure thereto. According to this process, only two layers of printed wiring boards can be laminated via an intermediate insulating plate having holes filled with conductive material for connecting the circuits. It is clear that this process cannot be applied to a process for producing a multilayer circuit board having 10 to 20 or more layers of substrates from the viewpoint of structure and workability.